The thing is the game is already balanced around action speed affecting...action speed. Making this also true in turn based mode isn't going to suddenly make it any more or less overpowered than it is in the real time mode.
The same tradeoffs that apply in real time mode would still apply in turn based mode - armor vs speed, might vs dex, fast weapons doing less damage per action and dual wield having an accuracy and damage penalty (which, by the way, needs to be bumped in Turn based mode anyway since single weapon is hilariously bad in a both-weapons-attack-in-one-action and one-action-per-round world).
They've got two options - find a way to apply existing balance to turn based mode (fairly easy) requiring minimal rebalancing. Or completely throw all previous balancing work out the window and make a completely new balance from scratch for the entire game for a fixed-rounds 1-action per round turn based mode. One of these is affordable and realistic, the other isn't.